Arboreal alligator lizards - yes, really

As you might guess from the following article, I still have a bit of a thing going on with anguid lizards (the family that includes slow-worms, glass lizards, alligator lizards and galliwasps). This is despite the fact that I spent a lot of time over the last few days talking about new tupuxuarid pterosaurs, the behaviour of hadrosaurs, and the discovery of multiple (yes, multiple) new large mammal species in the Amazon...

If you live in a northern place where anguids occur, you might regard these lizards as denizens of predominantly subtropical or temperate climates. The vast majority of species, however, are tropical. Some of them are really neat looking, if not beautiful and/or bizarre: many of them are very poorly known and many are in danger of extinction. In fact at least some have become extinct within recent decades. In view of all this it seems appropriate to bring these charismatic lizards to wider attention [image above shows Abronia graminea: named by Cope in 1864, it was one of the first members of its genus to be recognised. Image from here]

It is generally agreed that crown-group anguids can be grouped into a galliwasp clade (Diploglossinae), an alligator lizard clade (Gerrhonotinae) and a glass lizard clade (Anguinae), though the monophyly of both diploglossines and anguines has been questioned (Sullivan 1987, Macey et al. 1999). In previous articles I've looked at the anguines: my intention this time round was to look at both gerrhonotines and diploglossines. There's too much to say though, so I'm going instead to stick to one genus of one of these groups. That's a shame, as I really wanted to get through the galliwasps. This is a group that includes several fantastically pigmented island-endemics, some of which are very large, and some of which are either very endangered or extinct [adjacent pic shows Central American galliwasp Diploglossus monotropis, a common species in the exotic pet trade]. In some galliwasps the juveniles are patterned to apparently mimic local species of poisonous centipede (there are actually several cases within Squamata of juveniles mimicking poisonous or noxious arthropods), and one galliwasp - the Malpelo Island species D. millepunctatus - is special in that it forages among seabird colonies, eating chicks and faeces, and even mobbing adults and eating the prey that they disgorge. Anyway, more on them later, sorry. Incidentally, I've mentioned that anguids have a good fossil record: while the extant groups all have records extending back to the Eocene and Palaeocene, there are some additional groups that are known only from fossils. I'll write about them one day too.

But back to the living taxa: one of the most enigmatic and unusual of anguid taxa is Abronia Gray, 1838, a group of mostly arboreal, high-altitude gerrhonotines from Mexico and northern Central America. While I would forgive you for having never heard of them, this one genus constitutes about half of all extant gerrhonotine diversity. They're sometimes called arboreal alligator lizards, and appear most closely related to the Central American alligator lizards Mesaspis, with the fossil taxon Paragerrhonotus being even more closely related (Good 1988). Paragerrhonotus is from the Pliocene of California and Kansas: there are apparently some interesting biogeographical implications that result from this affinity, but I don't know what they are as they're only discussed (so far as I know) in David Good's unpublished Phd thesis (which I haven't seen) [adjacent pic shows Mesaspis monticola, from here].

So far as I can tell, Abronia itself doesn't have a fossil record, but a few fossil species referred to other gerrhonotine genera appear particularly close to it and might either be close to the ancestry of the group, or unrecognised stem-group Abronia species. Gauthier* (1982) and Estes (1983) noted that Gerrhonotus mungerorum from the Miocene and Pliocene of Nebraska and Kansas resembles some Abronia species, and might be close to this taxon (though they noted that it also resembles Barisia). Indeed, given that Paragerrhonotus, supposedly the closest relative of Abronia, is from California (with some questionable specimens from Nebraska), it is plausible that Abronia originated in what is now the USA before moving southwards during the Pliocene, perhaps as a result of the cooling conditions that occurred at this time. Note here that old cautionary tale about neglecting fossil species when looking at problems of this sort: Macey et al. (1999) produced a biogeographical analysis of gerrhonotines based on extant taxa and, because extant members of the closely related taxa Mesaspis, Abronia, Gerrhonotus and Barisia have distributions centred around Mexico, Texas and Central America, their area cladogram makes it look as if Mexico (or thereabouts) was the place of origin for this clade. Throw in the fossils, and that pattern changes.

* For those of you who know Jacques Gauthier for his work on theropod dinosaurs, I take pleasure in telling you that he was initially far, far better known for his work on lizards. He even has an iguana (Armandisaurus explorator Norell & de Queiroz, 1991) named in his honour [adjacent pic of Gauthier from his website. A prize to anyone who can identify at least four of the books on his shelves].

Abronia species usually have prominent, rugose osteoderms (though some species have a strongly reduced osteoderm compliment), and broad and flattened heads that, in some species (such as A. aurita and A. anzuetoi, both from Guatemala), sport short spikes around the ears. In contrast to other gerrhonotines, Abronia species have proportionally normal-sized limbs. Their tails are prehensile, but like other anguids they are still able to autotomise if necessary, with regenerated tails being shorter and less effective. They are mostly green (some are bluish or turquoise*, and some are brown), with some species possessing areas of yellow, orange or black. In some species the pigmentation forms patterns recalling patches of lichen or moss. All known species are viviparous, with some giving birth to single young, while others produce up to 12 babies.

* Captive individuals of some species have been observed to change colour over a period of a few months.

Field data shows that many Abronia species spend most of their lives living among epiphytes, and they take shelter in bromeliads and tree holes. Some species may live in tree-tops 40 m above the ground, whereas others are encountered beneath logs and rocks on the forest floor. Bogert & Porter (1967) noted that, where arboreal Abronia species occur in Oaxaca, Mexico, the sympatric Barisia and Gerrhonotus species are strictly terrestrial, but that, in Arizona and California (where there are no Abronia species), Gerrhonotus can be found in trees. Unexpectedly, an individual of A. fimbriata in Guatemala was reported swimming in a stream and repeatedly diving to the bottom.

Abronia was comprehensively reviewed by Campbell & Frost (1993): they recognised 23 species which they grouped into six subgenera (Abaculabronia, Abronia, Aenigmabronia, Auriculabronia, Lissabronia and Scopaeabronia), and even before they had completed their manuscript they knew of several additional species that had either been published, or awaited publication [A. oaxacae, shown here, belongs to the subgenus Abronia]. One species, named by Campbell & Frost (1993), has the neat name A. gaiophantasma: the specific name means 'spirit of the earth'. Sympatry among Abronia species is unusual and very rare, and reported cases have proved controversial and either erroneous, or just about impossible to verify (Campbell & Frost 1993, Townsend Peterson & Nieto-Montes 1996, Pianka & Vitt 2003).

The bad news is that the very localized distribution of many of the species makes them vulnerable to extinction, and it's been estimated that perhaps 13 species will become extinct in the next few decades. Indeed some species (e.g., A. mitchelli from Oaxaca, named in 1982) are known only from a single specimen, or have gone undocumented for several decades (e.g., A. montecristoi from El Salvador, named in 1983), so we have no information on their current status. The future is not bright for a taxon that contains about half of all extant gerrhonotine species.

Refs - -

Bogert, C. M., & Porter, A P. (1967). A new species of Abronia (Sauria, Anguidae) from the Sierra Madre del Sur of Oaxaca, Mexico. American Museum Novitates, 2279, 1-21.

Campbell, J. A. & Frost, D. R. 1993. Anguid lizards of the genus Abronia: revisionary notes, descriptions of four new species, a phylogenetic analysis, and key. Bulletin of the American Museum of Natural History 216, 1-121.

Estes, R. 1983. Handbuch der Paläoherpetologie. Part 10A. Sauria Terrestria, Amphisbaenia. Gustav Fischer Verlag (Stuttgart, New York), pp. 249.

Good, D. A. 1988. The phylogenetic position of fossils assigned to the Gerrhonotinae (Squamata: Anguidae). Journal of Vertebrate Paleontology 8, 188-195.

Gauthier, J. A. 1982. Fossil xenosaurid and anguid lizards from the Early Eocene Wasatch Formation, southeast Wyoming, and a revision of the Anguioidea. Contribution to Geology of the University of Wyoming 21, 7-54.

Pianka, E. R. & Vitt, L. J. 2003. Lizards: Windows the Evolution of Diversity. University of California Press, Berkeley.

Sullivan, R. M. 1987. Parophisaurus pawneensis (Gilmore, 1928), new genus of anguid lizard from the middle Oligocene of North America. Journal of Herpetology 21, 115-133.

Townsend Peterson, A. & Nieto-Montes, A. 1996. Sympatry in Abronia (Squamata: Anguidae) and the problem of Mario del Toro Avilés' specimens. Journal of Herpetology 30, 260-262.